Three problems on weight paint
What do you mean when you say that when you're in Weight Paint mode, nothing happens? You need to be in Object mode, select the armature, Shift select the object, switch to Weight Paint mode, and Ctrl left click on the bone you want and you should see the weight.
You have black color instead of blue because you have enabled the Zero Weight option in the Viewport Overlays panel. It allows you to see real zero weight, because a very low weight will be displayed blue.

If you move some bones it may not work because you've selected the Bone vertex group in the Armature modifier, remove it if you want all your bones to work:

The black dotted line indicates the parentage between the object and the armature, it goes from the origin of your object to the origin of your armature, but these 2 origins are not centered. So select the armature and go to the header menu &gt; Object &gt; Set Origin &gt; Origin to Geometry, and select the object and right click &gt; Set Origin &gt; Origin to Geometry: